* Work around MSVC issue with std::atomic initialization
MSVC seems to have a bug where it does not use constant initialization
for std::atomic, which ends up causing crashes during initialization.
This change introduces a workaround by putting the std::atomic inside a
union, which causes the compiler to use constant initialization for it.
* Added an AppVeyor test for static linking with MSVC
This has one important packaging change: the netstandard version now
depends (implicitly) on netstandard1.6.1 rather than on individual
packages. This is the preferred style of dependency, and shouldn't
affect any users - see http://stackoverflow.com/questions/42946951
for details.
The tests are still NUnit, but NUnit doesn't support "dotnet test"
yet; the test project is now an executable using NUnitLite. (When
NUnit supports dotnet test, we can adapt to it.)
Note that the project will now only work in Visual Studio 2017 (and
Visual Studio Code, and from the command line with the .NET Core
1.0.0 SDK); Visual Studio 2015 does *not* support this project file
format.
